Contributors

Overview

The datasheet for the DS3231 explains that this part is an "Extremely Accurate I²C-Integrated RTC/TCXO/Crystal". And, hey, it does exactly what it says on the tin! This Real Time Clock (RTC) is the most precise you can get in a small, low power package.

Most RTC's use an external 32kHz timing crystal that is used to keep time with low current draw. And that's all well and good, but those crystals have slight drift, particularly when the temperature changes (the temperature changes the oscillation frequency very very very slightly but it does add up!) This RTC is in a beefy package because the crystal is inside the chip! And right next to the integrated crystal is a temperature sensor. That sensor compensates for the frequency changes by adding or removing clock ticks so that the timekeeping stays on schedule

This is the finest RTC you can get, and now we have it in a compact, breadboard-friendly breakout. With a coin cell plugged into the back, you can get years of precision timekeeping, even when main power is lost. Great for datalogging and clocks, or anything where you need to really know the time.

Comes as a fully assembled and tested breakout plus a small piece of header. You can solder header in to plug it into a breadboard, or solder wires directly.

Pinouts

Power Pins:

Vin - this is the power pin. Since the RTC can be powered from 2.3V to 5.5V power, you do not need a regulator or level shifter for 3.3V or 5V logic/power. To power the board, give it the same power as the logic level of your microcontroller - e.g. for a 5V micro like Arduino, use 5V

Other Pins:

BAT - this is the same connection as the positive pad of the battery. You can use this if you want to power something else from the coin cell, or provide battery backup from a different separate batery. VBat can be between 2.3V and 5.5V and the DS3231 will switch over when main Vin power is lost

32K - 32KHz oscillator output. Open drain, you need to attach a pullup to read this signal from a microcontroller pin

SQW - optional square wave or interrupt output. Open drain, you need to attach a pullup to read this signal from a microcontroller pin

RST - This one is a little different than most RST pins, rather than being just an input, it is designed to be used to reset an external device or indicate when main power is lost. Open drain, but has an internal 50K pullup. The pullup keeps this pin voltage high as long as Vin is present. When Vin drops and the chip switches to battery backup, the pin goes low.

Arduino Usage

You can easily wire this breakout to any microcontroller, we'll be using an Arduino. For another kind of microcontroller, just make sure it has I2C, then port the code - its pretty simple stuff!

Oops i removed the Power wire from 5V to the Vin rail before taking this pic, don't forget it!

Connect Vin to the power supply, 3-5V is fine. Use the same voltage that the microcontroller logic is based off of. For most Arduinos, that is 5V

Connect GND to common power/data ground

Connect the SCL pin to the I2C clock SCL pin on your Arduino. On an UNO & '328 based Arduino, this is also known as A5, on a Mega it is also known as digital 21 and on a Leonardo/Micro, digital 3

Connect the SDA pin to the I2C data SDA pin on your Arduino. On an UNO & '328 based Arduino, this is also known as A4, on a Mega it is also known as digital 20 and on a Leonardo/Micro, digital 2

The DS3231 has a default I2C address of 0x68 and cannot be changed

Download RTCLib

For the RTC library, we'll be using a fork of JeeLab's excellent RTC library RTClib - a library for getting and setting time from an RTC (originally written by JeeLab, our version is slightly different so please only use ours to make sure its compatible!)

First RTC Test

The first thing we'll demonstrate is a test sketch that will read the time from the RTC once per second. We'll also show what happens if you remove the battery and replace it since that causes the RTC to halt. So to start, remove the battery from the holder while the Arduino is not powered or plugged into USB. Wait 3 seconds and then replace the battery. This resets the RTC chip.

Load Demo

Open up File->Examples->RTClib->ds3231 and upload to your Arduino wired up to the RTC

Upload to your Arduino and check the serial console @ 9600 baud. After a few seconds, you'll see the report that the Arduino noticed this is the first time the DS3231 has been powered up, and will set the time based on the Arduino sketch.

Unplug your Arduino plus RTC for a few seconds (or minutes, or hours, or weeks) and plug back in.

Next time you run it you won't get the same "RTC lost power" message, instead it will come immediately and let you know the correct time!

From now on, you wont have to ever set the time again: the battery will last 5 or more years.

Reading the Time

Now that the RTC is merrily ticking away, we'll want to query it for the time. Lets look at the sketch again to see how this is done.

There's pretty much only one way to get the time using the RTClib, which is to call now(), a function that returns a DateTime object that describes the year, month, day, hour, minute and second when you called now().

There are some RTC libraries that instead have you call something like RTC.year() and RTC.hour() to get the current year and hour. However, there's one problem where if you happen to ask for the minute right at 3:14:59 just before the next minute rolls over, and then the second right after the minute rolls over (so at 3:15:00) you'll see the time as 3:14:00 which is a minute off. If you did it the other way around you could get 3:15:59 - so one minute off in the other direction.

Because this is not an especially unlikely occurrence - particularly if you're querying the time pretty often - we take a 'snapshot' of the time from the RTC all at once and then we can pull it apart into day() or second() as seen above. Its a tiny bit more effort but we think its worth it to avoid mistakes!

We can also get a 'timestamp' out of the DateTime object by calling unixtime which counts the number of seconds (not counting leapseconds) since midnight, January 1st 1970

Since there are 60*60*24 = 86400 seconds in a day, we can easily count days since then as well. This might be useful when you want to keep track of how much time has passed since the last query, making some math a lot easier (like checking if its been 5 minutes later, just see if unixtime() has increased by 300, you dont have to worry about hour changes).

Notice the parameters to the struct_time initializer, you must pass in a tuple of all the values listed above.

Now if you read the datetime property you'll see the clock is running from the set time. For example if you want to read and print out just the year, month, day, hour, minute, and second you could run:

# Simple demo of reading and writing the time for the DS3231 real-time clock.
# Change the if False to if True below to set the time, otherwise it will just
# print the current date and time every second. Notice also comments to adjust
# for working with hardware vs. software I2C.
import time
import board
# For hardware I2C (M0 boards) use this line:
import busio as io
# Or for software I2C (ESP8266) use this line instead:
#import bitbangio as io
import adafruit_ds3231
i2c = io.I2C(board.SCL, board.SDA) # Change to the appropriate I2C clock & data
# pins here!
# Create the RTC instance:
rtc = adafruit_ds3231.DS3231(i2c)
# Lookup table for names of days (nicer printing).
days = ("Sunday", "Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday")
#pylint: disable-msg=bad-whitespace
#pylint: disable-msg=using-constant-test
if False: # change to True if you want to set the time!
# year, mon, date, hour, min, sec, wday, yday, isdst
t = time.struct_time((2017, 10, 29, 15, 14, 15, 0, -1, -1))
# you must set year, mon, date, hour, min, sec and weekday
# yearday is not supported, isdst can be set but we don't do anything with it at this time
print("Setting time to:", t) # uncomment for debugging
rtc.datetime = t
print()
#pylint: enable-msg=using-constant-test
#pylint: enable-msg=bad-whitespace
# Main loop:
while True:
t = rtc.datetime
#print(t) # uncomment for debugging
print("The date is {} {}/{}/{}".format(days[int(t.tm_wday)], t.tm_mday, t.tm_mon, t.tm_year))
print("The time is {}:{:02}:{:02}".format(t.tm_hour, t.tm_min, t.tm_sec))
time.sleep(1) # wait a second

# Simple demo of reading and writing the time for the DS3231 real-time clock.
# Change the if False to if True below to set the time, otherwise it will just
# print the current date and time every second. Notice also comments to adjust
# for working with hardware vs. software I2C.
import time
import board
# For hardware I2C (M0 boards) use this line:
import busio as io
# Or for software I2C (ESP8266) use this line instead:
#import bitbangio as io
import adafruit_ds3231
i2c = io.I2C(board.SCL, board.SDA) # Change to the appropriate I2C clock & data
# pins here!
# Create the RTC instance:
rtc = adafruit_ds3231.DS3231(i2c)
# Lookup table for names of days (nicer printing).
days = ("Sunday", "Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday")
#pylint: disable-msg=bad-whitespace
#pylint: disable-msg=using-constant-test
if False: # change to True if you want to set the time!
# year, mon, date, hour, min, sec, wday, yday, isdst
t = time.struct_time((2017, 10, 29, 15, 14, 15, 0, -1, -1))
# you must set year, mon, date, hour, min, sec and weekday
# yearday is not supported, isdst can be set but we don't do anything with it at this time
print("Setting time to:", t) # uncomment for debugging
rtc.datetime = t
print()
#pylint: enable-msg=using-constant-test
#pylint: enable-msg=bad-whitespace
# Main loop:
while True:
t = rtc.datetime
#print(t) # uncomment for debugging
print("The date is {} {}/{}/{}".format(days[int(t.tm_wday)], t.tm_mday, t.tm_mon, t.tm_year))
print("The time is {}:{:02}:{:02}".format(t.tm_hour, t.tm_min, t.tm_sec))
time.sleep(1) # wait a second

OUT OF STOCK NOTIFICATION

YOUR NAME

YOUR EMAIL

You have been successfully subscribed to the Notification List for this product and will therefore receive an e-mail from us when it is back in stock!

For security reasons, an e-mail has been sent to you acknowledging your subscription. Please remember that this subscription will not result in you receiving any e-mail from us about anything other than the restocking of this item.

If, for any reason, you would like to unsubscribe from the Notification List for this product you will find details of how to do so in the e-mail that has just been sent to you!